The present invention relates to a turntable for disc-shaped information carriers and the process for its production, the turntable having a central centering piece, which engages through a centring bore of the information carrier, and an outer marginal surface of the turntable, such as for example in the case of a CD player, which forms a bearing surface for the information carrier.
Disc-shaped information carriers, which for recording and reproducing information are rotatingly scanned or written to on a turntable, are generally known. While disc-shaped information carriers in the form of the long-known analogue phonographic record are generally placed in full surface contact on a turntable and rotate at a relatively low speed of 331/3 or 45 revolutions per minute, disc-shaped information carriers, such as for example the CD or MOD, for which the dense storage technique is used are placed or restrained in a central region bearing no information and rotate at a considerably higher speed in the range of, for example, 500 revolutions per minute. On account of the high speed and the small dimensions of the regions to be scanned and on account of the central restraint of the information carrier, only small deviations of the rotating information carrier from the intended scanning location in the radial direction and in the vertical position can be allowed, meaning that high accuracy requirements are imposed on the fixing and centring of the information carrier on the turntable and on the turntable itself.
Apart from the centring, the accuracy of which is influenced by the centring piece engaging through the centring bore of the information carrier, narrow production tolerances have to be maintained on account of the central restraint of the information carrier, in particular with regard to the runout in the vertical direction of the turntable, i.e. the vertical oscillation of the turntable during rotation since a vertical turntable runout of, for example only 0.05 mm, which represents a maximum permissible upper limit, already causes at the outer edge of a CD a vertical runout of 0.2 mm, which in many cases no longer ensures the scanning reliability required for the information carrier.
There is a known turntable for disc-shaped information carriers which has been used for many years in CD players and has a bearing bush of brass which is pressed onto the drive spindle of a motor and on which an internally offset, flat metal disc is riveted on or splined with the bearing bush and forms a bearing surface for the information carrier, cf. FIG. 3. On account of the tolerances of the riveting surface of the brass bush with respect to the bearing surface of the information carrier, often the maximum permissible vertical runout is not maintained and results in a high proportion of defective turntables and high numbers of rejects. In addition, the joining operation of riveting has an adverse effect on the precision of the turntable, and fastening of the brass bush on the motor spindle requires high pressing-on forces, which result in deformations which likewise contribute to the vertical runout and to centring deviations. In order to reduce the pressing-on force or the load-bearing surface, slots have already been provided in the brass bush, but have only achieved the effect of a further deterioration in the running properties of the turntable.
For centring the information carrier on the turntable, a centring cone which is displaceable in the direction of the spindle of the turntable and is formed by a third part is provided. Although the displaceability of the centring piece on the turntable spindle advantageously permits an adaptation to tolerances of the centring bores of the information carriers, it itself causes eccentricities or deviations of the position of the information carrier in the radical direction on account of the play of the centring piece with respect to the turntable spindle.
Although the moment of inertia or the flywheel moment is no longer attributed prominent importance on account of the electronic direct drive of the turntable, it should be mentioned that nevertheless a high moment of inertia or a great flywheel moment is desirable to improve the running properties. The metal parts essentially influencing the moment of inertia are distributed relatively uniformly over the overall turntable area, so that there is a substantially uniform mass distribution.
Furthermore, a turntable consisting of a single piece of plastic-bonded, permanently magnetic powder material for recording on disc-shaped information carriers is known, cf. German Utility Model 78 34 662 of 22.11.1978. For producing the plastic-bonded, permanently magnetic material, a complex technological process is required, which has an adverse effect on the costs of the turntable. The uncomplex connection of the turntable to the motor spindle by means of a press fit has the effect in the case of the proposed powder material of stress cracks, consequently leading to failure of the player. In addition, the production-dependent range of variation of one-piece turntables of plastic-bonded materials does not meet the imposed requirements for accuracy or dimensional stability. Tests with solid plastic versions have shown that, in spite of a plurality of gating points, the accuracy described above cannot be maintained over a lengthy period of time also on account of the flow properties of the plastics and production-dependent tolerances.
On account of the homogeneity of the material, the turntable has no features supporting the flywheel moment, which tend rather to be adversely affected due to the low weight of the one-piece turntable.
The centring piece has a steep cone, meaning that a relatively accurate prepositioning with respect to the turntable is required for centring the information carrier.
Also known in conjunction with a one-piece turntable is a centring piece which is additionally placed on the one-piece turntable and is formed by plastic springs, cf. Specification CDM 12.3 in N.V. Philips Industrial Activities, REV. 1.0 of 14 Sep. 1992, page 27, FIG. 10. Since the tolerances of the centring piece add to the tolerances which the bearing surfaces of the turntable already have with respect to the drive spindle of the motor for production reasons, and since plastic springs suffer from fatigue for only a short time, only a low centring accuracy can be achieved.
Also known is a turntable for disc-shaped information carriers which comprises a flat metal disc which is offset in the inner region and which has in the inner region a plastic body, by which the motor spindle is received, cf. Drive unit for compact disc player (KSM-210) in Sony Electronic Devices General Catalog '91, page 76. The outer, annular edge of the metal disc serves as a bearing surface for the information carrier and the plastic body is arranged in the inner portion of the metal disc at a distance from the inner edge of the surface intended as a bearing for the information carrier. The inner region of the metal disc has recesses, into which the plastic body engages to achieve an anti-twisting effect. As a sealing surface in the injection mould for fitting the plastic body into the metal disc, use is made of an offset inner surface of the metal disc at a distance from the bearing surface of the information carrier. By the use of a plastic body in the inner region of the turntable, lower pressing-on forces are required, in particular for mounting the turntable on the motor spindle. By replacing the brass bush mentioned at the beginning by a plastic body, it has not been possible however to achieve any significant improvements with regard to the vertical runout of the turntable, the centring accuracy or radial errors occurring and the moment of inertia. With an envisaged centring piece which is slidingly displaceable in the direction of the motor spindle on a surface of the plastic body, only a low centring accuracy can be achieved on account of the cumulative tolerances between motor spindle and plastic body and also between the plastic body and the centring piece and on account of the clearance existing between the plastic body and the centring piece.
The object of the invention is therefore to provide a turntable for disc-shaped information carriers which ensures by its very design and its production process a low vertical runout, a high centring accuracy and an advantageous moment of inertia in an uncomplex way.
This object is achieved according to the invention by the features specified in Claims 1 and 6 and by the advantageous designs specified in the subclaims.